Arrangement for the reproduction of sound



July 20, 1943. Q K, KOLB ET AL 2,324,781

ARRANGEMENT FOR THE REPRODUCTION OF SOUND Filed March 19, 1942.

Patented July 20, 1943 i d ARRANGEMENT FOR THE REPRODUCTION O'F SOUNDvOtto Kurt Kolb and David Whitehead Boston, f London, England, assignorsto British Acoustic Films Limited, London, England, a British companyApplication March 19, 1942, Serial No. 435,332 In Great Britain April 4,1941 7 claims. (o1. 17e-100.3)

fThis invention relatesto the recording and reproduction of sound withthefuse of sound film records. Picture iilms have been provided with oneor more sound tracks and witha control track adapted to controlamplilication duringl reproduction,` the control `track being arranged`within or substantially within the strip of the film occupied by thesprocket perforation holes "at one edge of the iilm. Each of the twoperforation strips of the lm may be provided with such a. control track.

According to the specification of British Patent No. 473,256,foriexample, the sound is recorded on the sound track under volume"compression, and the amount of compression is registered vby theamplification of the .reproducing amplifier.

Bythis arrangement an expansion of the volume of thefrecorded soundiseliected the reproductiontaking place L through on amplifier channel;

n 4Such an arrangement suffers from'one draw.-

` lback inasmuch as the maximum volume of undlstortedsound reproducibleis limited `by` the output capacity of the amplifier; This does notmatter for dialogue and quiet musical pieces which make up the greaterpart of a soundl accompaniment to a lm, but it does place a limitationon the artistic interpretation of `fully o`r chestrated musical passagesand on special ef- `fects such as gunfire, running engines and the like.The principal object of the presentinvention is to overcome thisdrawback by making use of a control current derived from a track ortracks in the sprocket hole line of the film to expand the volume rangeof the recorded sound in an" improved manner by the use of one or more`additional amplifierr channels and so to ,in-

crease the maximum -volume of sound repro!- diicible and td'enhance therealism ofthe reproduced sounds. l

" A further object is to enable a control current to be'derived from acontrol track-or tracks located within the sprocket hole strip of iilmvand to apply this control current tocontrol a further reproducingamplier `or ampliiiers ad'- dltional to the main reproducing ampliiier.

when one additional ampliner is provided, the output from thisadditional amplifier may be fed to the same loudspeaker or` loudspeakersas those to which the outputfrom the mein ampliiier is fed, or, it maybe fed to additional loudspeakers. vThe arrangement may also be suchthat the low frequency sound currents from the main and additionalamplifiers are all feti to the same low frequency loudspeaker Whilst thehigh frequency currents are fed to separate loudspeakers.

The additional amplifier need not have the same frequencycharacteristics as the main `amplifier, for example, it can be designedto deal with onlya `small range of frequencies. so that it may be ofsimple design and inexpensive 9.0111: struction. .Moreoverl theadditional amplifie! will preferably have a much greater power out.- putthan the main amplier. I

One feature of the invention resides 1n the provision of a control unitwhich is fed 'with current derived from the sound track Proper .011 thefilm and also with current derived from the control track or tacks, andis so arranged as either to control all the amplifiers, including themain amplifier, or only to bring in one or more additional amplifiers asand when required` The invention will now be described, by'way ofexample, with reference to the accompanying drawing, wherein-- J iFigure 1 shows a diagramof one embodiment according to the inventionwherein current from the sprocket hole control track controls both themain amplifier and an additional ampliiier.V

Figure 2 shows a diagram of a modification of the embodiment accordingto Figure 1 wherein only the additional Vampliiier is so contro11ed.`

Figure 3 illustrates an embodiment of the cir:` cuit arrangement of thecontrol unt'shown m Figure l, and l n n Figure 4 illustrates anembodiment ,of the cir,- cuit arrangement of the control unit showninFigure 2. i i A n Figure 5 shows a small portion of a film with thesound and .control tracks thereon. l f i Referring to Figures ,il andj2, C represents the conventional light-sensitive cell and` opticalsyse' tem and `HA representsv the normal head ame p'lifier such as isfitted to 'present day sound lm projectors. :This amplier is used toamplify photo-electrically produced sound l currents obthe sound track 3adjacent the sprocket holes at the same side of the film. CI representsa light sensitive device, such as a photo-electric or selenium cell,which, together with its optical system, is used to produce a controlcurrent from the control track in the line of the sprocket holes on thefilm.

A further cell and optical system, C2, is also shown in dotted lines;this cell can be used to produce a control current from a second controltrack, if one is provided vin the line of the second set of sprocketholes 2 on the film.

The outputs from the devices HA and CI are fed to a collecting unit orfader F. Such a fader is provided for the manual adjustment of the soundfilm projector and also for facilitating changing over from oneprojector to another as is necessary at the end of each lm reel.

In Figure 1, both the currents fed to the fader F are passed on to acontrol unit CU which controls both the main amplifier MA and anadditional amplifier AA which in turn supply current to the loudspeakers LS.

In Figure 2, it Will be seen that the control current fed to the fader Fis passed on to the control unit CU, as in Figure 1, whilst the soundtrack current is passed straight on to the main amplifier MA. A portionof this current from the head amplifier HA is, however, branched offfrom the connection between the fader and the main amplifier and fed tothe control unit which thus only controlsrthe additional amplifier AA.

' In the arrangements shown in Figures l and 2 of the drawingv thecontrol unit CU brings in the additional amplier AA at a certainamplitude ofthe control track and controls its amplification over apredetermined amplitude range. The outputs from the amplifiers MA and AAcan be fed to the same or parallel loudspeakers LS; in fact, anyYdesired arrangement of speakers can be employed. Figure 3 illustratesan embodiment of the circuit arrangement of the control unit CU shown inFigure 1. A `light beam modulated by the sound track 3 of Figure 5 isfocussed :on the light sensitive cell C, which may be ayphoto-electriccellor-selenium cell, and the output of the cell C is fed. to the headamplifier HA, the output from which is fed to the fader F. A light beammodulated by the control track I of Figure 5 is focussed on the lightsensitive cell CI and the output from this cell, after amplification ifde-. sired, is also fed to the fader F. From the fader it is passed. on.to the first valve Vi of the control unit.l

It will be appreciated that the light beam focussed on the cell CIconsists of a modulated beam having a carrier frequency of 96 cycles persecond, since at normal film feeding speed, 24 picture frames andtherefore 96 sprocket holes will pass through the light beameverysecond. Thetransformer TRI, fed with the output of the valve VI, istherefore tuned to this frequency. The output from the transformer TRIis rectified by a rectifier RI, smoothed by a filter circuit FCI and fedthrough the secondary of a transformer TR2 to bias the contro-l grid ofa Variable mu valve V2. The transformer TR2 is also connected to thefader F and fed with the output from the head amplifier HA. The outputfrom the Valve V2 is fed via a transformer TRS tothe main amplier MA, ofusual design, the output of which is thus controlled in dependence uponthe output of the said Valve.

The control grid of the valve V2 is biassed, as

for example by the battery BI, via a rectifier R2, the arrangement beingsuch as to limit the maximum amplification gain of the valve. As aresult, overloading of the valve with consequent distortion can beavoided and the valve can be made to handle currents only up to adesired maximum.

A portion of the control current is also fed via the rectifier RI, asecond filter circuit FC2 and a further transformer TRII to the controlgrid of a further valve V3, which is also of the variable mu type.

The variable mu valve V3 is also fed with a portion of the signalcurrent from the head amplifier via the fader and the transformer 'I'R4and it is also biassed by the rectifier R3 and battery B2 in a similarmanner to the valve V2 but to a different point on its workingcharacteristic so that it only begins to function, for example, afterthe valve V2 has become loaded to its predetermined maximum. The outputfrom the valve V3 is fed to` a transformer TR5 and from there it ispassed on to the additional amplifier AA, the output of which is thuscontrolled in dependence upon the output of the valve V3. The linesdesignated IIT+ and HT- at their right hand ends represent the leads ofthe unit connected to the plus and minus terminals of a service of hightension electric current (not shown) as is usual in amplifier units.

Reference has already been made to the feature that the additionalamplifier may be of simpler and less expensive construction than themain amplifier. In such a case it may occur that it gives rise tonoticeable background noise during those periods when it is notreproducing sound passages, for example, when the main amplifier isreproducing only pianissimo passages. The valve V3 controlling theadditional amplifier may therefore be provided with a switch orelectrical relay, operated by hand or otherwise, to bring it into useonly during those periods when itisunder the control of the controltrack. For example, an automatically operating electrical relay isindicated by the reference RL in Figure 3. This relay has a coil inseries with the anode of valve V3, and is arranged to close its contactas soon as the valve V3 begins to operate but to open when the valve isnot operating. As shown by way of example and in dotted lines, the relayRL may be used to open and close the output circuit of the additionalamplifier AA, leading to a loudspeaker LS.

Figure 4 illustrates an embodiment'of the circuit arrangement of thecontrol unit' CU shown in Figure 2. The arrangement so far as the headamplifierHA, control cell CI land fader F are concerned is the same asin Figure 3. However, in this embodiment, the signal currents frornthehead'amplifier HA are fed straight on to the main amplifier MA via thefader F and, consequently, the main amplifier does not come under'thecontrol of the control unit. A part of the signal current from the headamplifier is branched off from the connection between the fader and themain amplifier and fed'to the control unit, that isgjto say, thetransformer TR2. l The control current is obtained as above4 means ofthe valve VI, tuned transformer 'IRI, rectifier RI, iilter circuit FCIand transformer TR2 and applied to the control grid of the variable muvalve V2. This valve is also biassed by battery Bl Via aI rectifier R2so as to limit the maximum amplification gain. The output fro-m thevalve V2 is fed to the transformer TR3 and from there is passed on toythe additional amplier AA instead of to the main amplifier as in Figure3; The circuit of valveVz may also be provided with a switching relay RLif desired, as in Figure 3. i

By the arrangement according to Figure 4, only the additional amplifieris controlled by the lcontrol track, and since this additional amplifiermay have a maximum output several times that of the main amplifier, itwill be appreciated that it is possible to employ it fo-r thereproduction of special effects, fully orchestrated musical pieces andthe like, with great advantage.

Whilst in the foregoing specific embodiments of the invention thecontrol of the amplifiers depends upon the amplitude of the controltrack I, it Will be understood that the control :can be effected bymeans of different frequencies carried by the control track.

We claim:

1. Apparatus for reproducing sound from film records such as areprovided with at least one control track in line with the sprocket holesof the film in addition to the sound record track thereon, saidapparatus comprising means for deriving aY sound current from said soundrecord track, means for deriving a control current from said controltrack, a main amplifier and an additional amplifier, a control unitincluding a first amplifying valve, means for applying said con-l trolcurrent to the grid of said valve, a transformer tuned to a frequencycorresponding to the rate at which ,the sprocket holes in the film whichare in line with the control track pass through the apparatus means forapplying the f output of said valve'to saidtransformer, a rectifierconnected to the output of said transformer, `a second valve of a typeadapted for variably controlled amplification, means for applying theoutput of said rectifier as a grid bias control to a grid circuit ofsaid second valve, means for applying sound current to the input of saidsecond valve, and means for applying the output of said second valve tothe input of said additional amplifler.

2. Apparatus for reproducing sound from nlm records such as are providedwith at least one control track in addition to the sound record trackthereon, said apparatus comprising means for deriving a sound currentfrom said sound record track,.means for deriving a control current fromsaid control track, a main amplifier and an additional amplifier, acontrol unit including a first amplifying valve, means for applying saidcontrol current to the grid of said valve, a transformer and means forapplying the output of said valve to said transformer, a rectifierconnected to the output of said transformer, a second valve of a typeadapted for variably controlled amplification, means for applying theoutput of said rectifier as a grid bias control to a grid circuit ofsaid second valve, a constant vo-ltage source and means including arectifier for applying a constant voltage therefrom to said grid circuitwhereby the gain of said second valve may be limited, means for applyingsound current to the input of said second valve, and means for applyingthe output of said second valve to the input of said additionalamplifier.

3. Apparatus for reproducing sound from film records such as areprovided with at least one control track in addition to the sound recordtrack thereon, said apparatus comprising means for deriving a soundcurrent from said record track, means for deriving a control currentfrom said control track, a main amplifier and an additional amplifier, acontrol unit including a first amplifying valve, means for applying saidcontrol current to the grid of said valve, a transformer and means forapplying the outputof said valve to said transformer, a rectifierconnected to the output of said transformer, a second valveof a typeadapted for variably controlled amplification, means for applying theoutput of said rectifier as a grid bias control to a grid circuit ofsaid second valve, meansl for applying sound current to the input ofsaid second valve, means for applying the output of said second valve tothe input of said main amplifier, a third valve also of a type adaptedfor variably controlled amplification, means for applying grid biascontrol from` the output of said first valve to a grid circuit of saidthird valve, means for applying sound current to the inputof said thirdvalve, and connections for applying current from `the output of saidthird valve to the input of said additional amplifier. l

4. Apparatus for reproducing sound from film records such as areprovided with at least one control track in addition to the sound recordtrack thereon, said apparatus means for deriving a sound current from.sai record trackfrneans for deriving a control current from lsaidcontrol track, a main amplifier 'and an additional amplifier', a controlincluding a first amplifying valve, means for applying said contro-lcurrent to the grid of vaive, a transformer and means for applying theoutput of said valve to saidtransformer, a rectifier `connected to theoutput of said transformer, a second valve of a type adapted forvariahly controlled amplification, means for applying the output of saidrectifier as a grid bias control to a grid circuit of said second valve,inea-iis for applying sound current to the input of said second valve,means for applying the output of said second valve to the input of saidmain amplifier, a third valve also of a type adapted for variablycontrolled amplification, means for applying grid bias control from theoutput of said first valve to a grid circuit of said third valve, meansfor applying sound current to the input of said third valve, andconnections for applying current from the output of said third valve tothe input Vof said additional amplifier, a constant voltage source andmeans including a rectifier for applying a constant voltage therefrom tothe grid circuit of said second valve whereby the gain of said valve maybe limited, and means for applying a grid bias from such a constantvoltage so-urce also to the grid circuit of said third valve.

5. Apparatus for reproducing sound from film records such. as areprovided With at least one control track in addition to the sound recordtrack thereon, said apparatus comprising means for deriving a soundcurrent from said record track, means for deriving a control currentfrom said control track, a main amplifier and an additional amplifier, acontrol unit including a first amplifying valve, means for applying saidcontrol current to the grid of said valve, a transformer and means forapplying the output of said valve to said transformer, a rectier connected to the output of said transformer, a second valve of a typeadapted for variably controlled amplification, means for applying theoutput of said rectifier as a grid bias control to a grid circuit ofsaid second valve, means for applying sound current to the input of saidsecond valve, means for applying the output of said second valve to theinput of said main amplier, a third valve also of a type adapted forvariably controlled ampliiication, means for applying grid bias controlfrom the output of said first valve to a grid circuit of said thirdvalve, means for applying sound current to the input of said thirdvalve, and connections for applying current from the output of saidthird Valve to theiinput of said additional amplifier, a constantvoltage source and means including a rectifier for applying a constantvoltage therefrom to the grid circuit of said second valve whereby thegain of said valve may be limited, and means for applying a grid biasfrom such a constant voltage source also to the grid circuit of saidthird valve, the value of the bias applied to the grid of said thirdvalve being such that its coming into operation is delayed until afterthe second valve has been loaded to a substantial extent.

6. Apparatus for reproducing sound from film records such as areprovided with at least one control track in addition to the sound recordtrack thereon, said apparatus comprising means for deriving a soundcurrent from said sound record track, means for deriving a controlcurrent from said control track, a main amplier and an additionalamplier, a control unit including a first amplifying valve, means forapplying said control current to the grid of said valve, a transformerand means for applying the output of said valve to said transformer, arectier connected to the output of said transformer, a second valve of atype adapted for variably controlled amplification, means for applyingthe output of said rectifier as a grid bias control to a grid circuit ofsaid second valve, means for applying sound current to the input of saidsecond valve, means for applying the output of said second valve to theinput of said additional amplier, and an electrical relay in the outputcircuit of said second valve adapted to bring said additional amplifierinto operation only When current is flowing in the output circuit ofsaid second valve.

7. Apparatus for reproducing sound from film records such as areprovided with at least one control track in addition to the sound recordtrack thereon, said apparatus comprising means for deriving a soundcurrent from said record track, means for deriving a control currentfrom said control track, a main amplier and an additional amplier, acontrol unit including a iirst amplifying valve, means for applying saidcontrol current to the grid of said valve, a transformer and means forapplying the output of said valve to said transformer, a rectiiierconnected to the output of said transformer, a second valve of a typeadapted for variably controlled amplification, means for applying theoutput of said rectier as a grid bias control to a grid circuit of saidsecond valve, means for applying sound current to the input of saidsecond valve, means for applying the output of said second valve to theinput of said main amplifier, a third valve also of a type adapted forvariably controlled amplification, means for applying grid bias controlfrom the output of said first valve to a grid circuit of said thirdvalve, means for applying sound current to the input of said thirdvalve, and connections for applying current from the output of saidthird valve to the input of said additional amplier, said connectionsincluding an electrical relay adapted to bring said additional amplifierinto operation only when current is flowing in the output circuit ofsaid third valve.

OTTO KURT KOLB. DAVID WHITEHEAD BOSTON.

